Our investigations of the innate immune response against West Nile virus (WNV), led to the discovery that WNV replication inhibits the signal transduction pathways used by interferons to induce cellular antiviral programs. The results indicated that one or more viral non-structural (NS) proteins inhibit the phosphorylation and activation of the Janus kinases JAK1 and Tyk2, which is required to establish an antiviral state in response to IFN. The purpose of the proposed research program is to investigate the mechanism responsible for this inhibition of the IFN response through three specific aims. [unreadable] [unreadable] In the first aim, we will use a genetic approach to identify the viral protein(s) that are responsible for the observed inhibition of the IFNa response in WNV infected cells. Our goal is to identify replication competent WNV mutants that have lost the ability to inhibit the IFN response. With a biochemical approach described in the second aim, we will investigate the host-virus interactions that play a role in the inhibition of the IFN signal transduction pathway in WNV infected cells. Specifically, we will determine if viral proteins inhibit the IFN response directly through binding to the IFNa receptor complex or if they operate indirectly by activating a cellular antagonist, such as a protein tyrosine phosphatase or other known negative regulators of the IFN response. Experiments were designed to identify the cellular factors that provide targets for the viral antagonists. With the third aim, we will investigate how the relationship between the host innate immune response and viral components influences the outcome of WNV infections. With the help of a system permitting the conditional expression of IFN-induced genes, we will investigate how cellular antiviral mechanisms inhibit WNV replication and determine the nature of the cellular genes that constitute the innate antiviral program against WNV. [unreadable] [unreadable] WNV has emerged as a pathogen of global significance that can cause paralysis, meningo-encephalitis, and death. Primary WNV infections cannot yet be prevented by vaccination and, so far, treatments to prevent the serious consequences of infections that affect particularly the elderly are not yet available. The studies proposed here will provide a better understanding of host-virus interactions that control WNV replication and pathogenesis. This new information can be used as the basis for the development of effective antiviral therapies. [unreadable] [unreadable] [unreadable]